Thermoelectricity



`une 23, 1964 Original D. E. HILL ETAL THERMOELECTRICITY Filed Nov. 27,1959 2 Sheets-Sheet l INVENTORS DALE E. HILL ARNOLD EPSTEIN ATTORNEYJune 23, 1964 D. E. HILL ETAL 3,138,485

THERMOELECTRICITY Original Filed Nov. 27, 1959 2 Sheets-Sheet 2 T HERMOELECTRIC GENERATOR.

WWWW//WWW/W//W/MWWWWMWWM INVENTORS DALE E. HILL AND BY ARNOLD EPSTEIN3,138,486 THERMGELECTRICITY Dale E. Hill, Kirkwood, and Arnold Epstein,St. Louis,

Mo., assignors to Monsanto Company, St. Louis, Mo.,

a corporation of Delaware l Original application Nov. 27, 1959i, Ser.No. 855,592. Divided and this application Aug. 31, 1962, Ser. No.

Claims. (Cl. IE6-5) The present invention relates to the use ofcrystalline boron phosphide for the direct conversion of heat toelectricity as well as processes for heating and cooling.

It is an object ofthe present invention to provide thermoelectricmaterials for obtaining electricity from heat sources, particularlyabove 800 C. It has been found that the prior art materials which havethermoelectric generating properties cannot be employed at such elevatedtemperatures `because of decomposition and the loss of usefultherrnoelectric properties.

Crystalline boron phosphide is hard, thermally stable and chemicallyinert.

The above-described boron phosphide may be prepared by a chemicalreaction between elemental boron and elemental phosphorus, or in generala boron source and a phosphorus source.

It has also been found that boron phosphide is characterized by unusual`thermal stability, e.g. low dissociation pressure. This is indicativeof a high order of stability at elevated temperatures, so that thismaterial can be used in thermoelectric applications withoutdecomposition. t

In using the hereindescribed composition for thermoelectric processes,the following electrical relationships and the data presented hereinshow the advantages of boron phosphide. The so-called figure of merit,Z, is delined as the ratio of the Seebeck coeflicient or thermoelectricpower, S,\squared to the product of the electrical resistivity, p andthermal conductivity K (Semiconductor Thermoelements and ThermoelectricCooling, p. 1,'A. F. Ioiie, Infosearch Limited, London (1957).) Theligure of merit, Z, can be seen to playan important role inthermoelectric devices used for heating, cooling, and power generation.In thermoelectric power generation, the theoretical maximum eiciencyobtainable is related to Z in the following way:

TI-TOXw/lJfitZwlJrTo-i T1? vlafazrrlwo-tg Etliciency where T1=temp. athot junction T0=temp. at cold junction (-Ibid., Part l, Chapt. 2, 13.40,A. F. Ioe.)

InV the drawings ofthe present case i `FIGrl 3 shows a thermoelectricdevice `using boron i phosphide as thermoelectric elements.

3,138,486 Patented Jun-e 23, 1964 In FIG. l, the property ofthermoelectric power is shown in relation to temperature for a number ofthermoelectric materials. The upper portion of the drawing includesP-type BP. It has been found that P-type BP compositions can be obtainedby the presence or inclusion of P-type impurities such as zinc, mercury,cadmium, beryllium or magnesium. For comparison, a conventional P-typematerial, Chromel alloy, is also shown. The lower portion of FIG. 1represents the N-type materials. The compound BP may be modified toobtain N-type characteristics by doping with an additive such as sulfur,selenium or tellurium. It is seen that a greater thermoelectric power isobtained with BP, particularly at higher temperatures, thanA with thecomparison materials indium arsenide, `InAs, or the mixed `binary indiumarsenide phophide, InAs 9P0 1.

In FIG. 2, the merit factor, Z, as discussed above, is shown for certainthermocouple combinations. The reference materials Chromel-constantanare` shown to have considerably inferior merit factors than the coupleof Chromel N-type BP, and the couple of N-type BP with P-type BP.

FIG. 3 illustrates a thermoelectric device. In FIG. 3 the P-typethermoelement such as P-type BP is indicated by the numeral 2 and theN-type material such as N-type BP by the numeral 3. These twothermoelements are` joined by a conventional conductor such as a copperbar, 6. Conventional conductors including soldered or welded joints arealso employed as elements 7 and 8 to connect to an external circuitexemplified by leads 9 and 10. In the operation of this device, a heatsource,.1, maintains the upper or reference junctions of ends of bars 2and 3 at an elevated temperature, while the lower ends or referencejunctions of bars 2 and 3 are kept cool by heat sinks 4 and 5. Thissystem yieldsa direct current which can be delivered to the electricalload, 11, connected in series or parallelin order to obtain largerelectrical effects. j j

In addition to the compound BP described above, the use `of variousmodified or doped compositions is included in the present invention,whether these modiliers are chemically combined or physically dispersed,e.g. in the space lattice of the base material. When one adds a smallamount, c g. 5% of carbon to the BP, one finds that the merit factor, Z,of the material is improved. The preferred group of elements formodifying or doping the boorn phosphide is carbon, arsenic, antimony,nitrogen (as nitrides), silicon, germanium, aluminum, gallium, sulfur,tellurium, selenium, zinc, cadmium, mercury, nickel, beryllium,magnesium, iron, palladium, platinum, tungsten, molybdenum and tantalum.

The saidelements are used as such and in compound forms, bothstoichiometric and non-stoichiometric, for' use as dopants or modifyingagents. Theproportion of the doping additive added to the above base ofboron phosphide as the dopants or additives is broadly in theV range ofless than 15% by weight, orV preferably from 0.005% to 15 by weight.` Astill more preferred range is from 0.01% to 10% `by weight relative tothe weight of the boron phosphide base material.

The mechanism by which modification of the` thermoelectric properties isobtained by doping has not been com-l pletely elucidated. However, ithas been found that theminute additions (1014to 10,1'7 carriers `per cc.ofV the matrix composition, that is from 0.00000l% to 0.001% by weight)of additives or` dopantscharacteristic of typical semi-conductorcompositions, e.`g., in transistors, rectitiers and diodes, are noteiective in the present thermo-` phide matrix).

, In general, the components of Vsuitable leads to .the external load.An example of such adsense c f, at

factorwhich are obtained with boron phosphide, BP, as the base material.f

' Composition Merit Factor, Z

BP Less than 1 105 BP with 0.0001% S l Less than 1)(105 BP With 1% S Inaddition to the improvementrof boron phosphide doping, it is alsopossible to improve the merit factor by the formation of mixed binarycompositions. As an instance ofv such a modified composition, it isfound that the'merit factor for the composition BPXAS1 X isimproved-relative to BP. vThe best improvement occurs vwhen the arsenicVto phosphorus ratio is greater than one;

lthe non-stoichiometric materials having the dopants dispersed therein(eg. in the-space lattice. of the boron phos- The followingexamplesillustrate specific embodiments of the present invention:

AAn example of the use of boron phosphide, BP, as anl ,Y element of athermocouple 'is shown inthe following exj ample.

Thev boron phosphide vas a block having N-type conductivity (by theaddition of 1% Te) is electrically joined to metallic Chromel Wire asthe P-type component. When this electrical junction is heated to atemperature of 1,0009 C., with the reference junctions at roomtemperature, an electromotive force ofy 0.45 volt is obtained. Thiscorresponds, with these particular materials, to a merit factor of0.4)(10-3 Q K."1.

a circuit isshown in PIG. 3. The electrical leads should be of goodelectrical and thermal conductivity.

I vExam ple 2 example of the use of N and P-type boron phos- .phide forarthermoelrectric coupleis shown in the following experiment.

A boron phosphide block having N-type conductivity v(by the addition of1% sulfur) is electrically joined at one end to a P-type boron phosphideblock (by the addition of11-% cadmium). When this junctionis lheated'toa temperature of 1,000 C. and the 'reference junctions at the remoteends of the two blocks are maintained at room temperature,anelectromotive force `of 0.8 volt is obtained. :This corresponds withthese `particular materials f to a meritfactor of 0.5 5 103 Krl.

. .y V.'.Example n '.,Inorder vtoobtain a cooling (or heating) effectfrom thet/thermocouple.,system yof Example 2, ten of the said couplesare-.joined'in series. A current of l0 amperes at V2, volts is passedthrough the system with the reference junctions'rnaintainedat 900 C. Itis found that the cooled (or-warmed) junctions reach a temperature ofabout 20 Clbelow (or above, respectively) the reference junction jvtemperatuge. .n

In order to comparethe ithermoelectric properties of boron phosphide toother compositionspthe following data shows the values for thethermoelectric power and also fofv the 'meritfactor and optimumoperating temperature Greater thanv x10-5 the couple arey joined by` irepresentative of thermoc'ouples based upon the following pairs:

Thermo- Merit Optimum electric Factor Operating Thermocouple MaterialoWer (Z 10%* Temp.,

(microvolts/ K.1) C.

N and P type BP 800 0.55 1,000 N type BP and Chromel... 4.50 0.4 1,000 Ptype BP and constantan. 450 0. 4 1,000

l It is a particular advantage of the boron phosphide, BP, that thismaterial maintains the desirable thermoelectric properties at unusuallyhigh temperaturessuch as in theV range of from 400 C. to 1,500" C. Theprior art thermoelectric materials have been found'to losetheir usefulthermoelectric properties very radically in this high temperature range.v v

The preparation of a' doped composition, such as BP modified by sulfuris readily conducted by conventional methods. For example yvvhenfsulfuris used as the dopant,

With BP, the sulfur is `dispersed in the space lattice of the'V BP bymixing the sulfur with the elemental boronand phosphorus beforereactingthese components to Vform BP.

Other methods include diffusion from vapor, VVliquid or solid additionsinto the base of boron phosphide. An-

other method is hot pressing, suitable, for example, inY

adding carbon to BP.

This application is a divisional application of copending US.applicationV Serial No. 855,592,1iled November 27, 1959, now abandoned.lu

What is claimed is: y 1. A thermoelectric generating materialvconsistingof boron phosphide, BP, containing dispersed therein as a n'modifying element'from 0.005% 'to 15% by weightrof at least onememberselected from the group consisting of carbon, arsenic, antimony,nitrogen (as nitrides), silicon, germanium, aluminum,'gallium, sulfur,tellurium, selenii um, zinc, cadmium, mercury, beryllium,l magnesium,

nickel, iron, palladium, platinum, tungsten, molybdenum,

and tantalum.

2. A thermoelectric generating material Yconsisting of boron phosphide,BP, containing dispersed'therein ,asv a modifyingy element, of 1% byweight.

3. A thermoelectric generating material consistingof boron phosphidehaving the formula BP containing dispersed therein as a modifyingelement, carbon which is present in the range of from 0.005 to 15% byWeight.

, l4.'A thermoelectric generating material 'whichy gener- Y ateselectricity at temperatures from 800 C, to 1500 C: consisting ofV boronphosphide, Bl'Easl a matrix containing from 0.005 to 175% by weight'ofatleast one member selectedV fromk the group consisting ofcarbongarsenic,

antimony, nitrogen (as nitrides), silicon, germanium,'alu j mmum,gallium, sulfur, teliurium, selenium, Zinc, lcadmium, mercury,beryllium,magnesium, nickel, iron, palla- Y dium, platinum, tungsten, molybdenum,and tantalum.

5. A thermoelectric couple suitable vfor use as an electric generatorwhich generates electricity at temperatures of from 800 C. to 1500 C.which comprises asa firstbranch of said couple boron phosphide, BP, hav'ingdis- 'persedtherein from 0.005% to 15% 'by .weight of. at

least one member selected from the group consisting of carbon,arsen1c,.ant1mony, vnitrogen (asnitrides),fsilicon,`

germanium, aluminum, gallium, sulfur,..tellurium, selernium, zinc,cadmlurn, mercury, beryllium, magnesium,

nickel,'iron, palladium, platinum, tungsten, molybdenum,

' and tantalum, and as a second branch of said couple boron Aof at leastone member selected from saidl group and Y '6. The thermcouple of claim5 wherein said-lii'rstk branch consists of boron phosphide, BP, Yhavingdispersed`V phosphide, P, having dispersed thereinr a likeamount havingopposite conductivity type as saidiirst branch, and electricalleadsattached to said couple; l

sulfur which is present in the amount therein 1% by Weight of sulfur toproduce N-type conductivity and said second branch consists of boronphosphide, BP, having dispersed therein 1% by weight of cadmium toproduce P-type conductivity.

7. Process for the production of electricity which comprises applyingheat at temperatures from 400 C. to 1500 C. to one end of athermoelectric couple while cooling the other end thereof, withdrawing acurrent from the said thermocouple, the said body having at least oneelectrical component composed of boron phosphide, BP, having dispersedtherein from `0.005 to 15% by weight of at least one member selectedfrom the group consisting of carbon, arsenic, antimony, nitrogen (asnitrides), silicon, germanium, aluminum, gallium, sulfur, tellurium,selenium, zinc, cadmium, mercury, beryllium, magnesium, nickel, iron,palladium, platinum, tungsten, molybdenum, and tantalum.

'8. Process for the production of electricity which comprises applyingheat at temperatures of from 400 C. to l5 00 C. to one end of athermoelectric couple consisting of boron phosphide, BP, of N-typeconductivity, by the addition of 1% sulfur, while cooling the other endwhich consists of boron phosphide, BP, of P-type conductivity, by theaddition of 1% cadmium, and withdrawing a current from the saidthermocouple.

9. Process for cooling a medium which comprises contacting the saidmedium with an end of a thermoelectric couple in which at least oneelectrical element is composed of boron phosphide, BP, having dispersedtherein from 0.005 to 15% by weight of at least one member selected fromthe group consisting of carbon, arsenic, antimony, nitrogen (asnitrides), silicon, germanium, aluminum, gallium, sulfur, tellurium,selenium, zinc, cadmium, mercury, beryllium, magnesium, nickel, iron,palladium, platinum, tungsten, molybdenum and tantalum, the saidthermoelectric couple having electrical leads connected thereto, andpassing a polarized electric current through the said couple whereuponthe end of the body is cooled and the medium in contact therewith iscooled.

10. Process for heating a medium which comprises contacting the saidmedium with an end of a thermoelectric couple in which at least oneelectrical element is composed of boron phosphide, BP, having dispersedtherein from 0.005 to 15% by weight of at least one member selected fromthe group consisting of carbon, arsenic, antimony, nitrogen (asnitrides), silicon, germanium, aluminum, gallium, sulfur, tellurium,selenium, zinc, cadmium, mercury, beryllium, magnesium, nickel, iron,palladium, platinum, tungsten, molybdenum and tantalum, the saidthermoelectric couple having electrical leads connected thereto, andpassing a polarized electric current through the said couple whereuponthe end of the body is heated and the medium in contact therewith isheated.

References Cited in the le of this patent UNITED STATES PATENTS2,798,989 Welker July 9, 1957 2,858,275 Folberth Oct. 28, 1958 2,921,973Heihes et al. Ian. 19, 1960 2,953,616 Pessel et al Sept. 20, 1960FOREIGN PATENTS 748,847 Great Britain May 6, 1956

1. A THERMOELECTRIC GENERATING MATERIAL CONSISTING OF BORON PHOSPHIDE,BP, CONTAINING DISPERSED THEREIN AS A MODIFYING ELEMENT FROM 0.500% TO15% BY WEIGHT OF AT LEAST ONE MEMBER SELECTED FROM THE GROUP CONSISTINGOF CARBON, ARSENIC, ANTIMONY, NITROGEN (AS NITRIDES), SILICON,GERMANIUM, ALUMINUM, GALLIUM, SULFUR, TELLURIUM, SELENIUM, ZINC,CADMIUM, MERCURY, BERYLLIUM, MAGNESIUM, NICKEL, IRON, PALLADIUM,PLATINUM, TUNGSTEN, MOLYBDENUM, AND TANTALUM.